It sounds as if you've got a plan (and oh, won't you both be having fun on Christmas day with both of you getting iPads!

).
If you set the iPads up so that you have Automatic Downloads turned OFF (it'll be in your Settings app, under the "iTunes & App Stores" tab), then your number three scenario will be the one that applies for you two. You two can use your Apple ID (and password) to download stuff - yet the other doesn't have to put it on their iPad.
If I may be so bold as to offer up two [more] suggestions:
(1) You both can have a version of iTunes on your own computer. As long as its authorized under the Apple ID you will share, then you can use iTunes to manage your purchases and such. You each will sync the iPad to your own computer (and remember, once it's synced to an iTunes/computer, you don't want to sync it to another - makes things hard to keep straight). The good thing is that you won't have her music in your iTunes (unless you want) and vice versa. But, with your own iTunes/computer, you each can manage your iPad how you wish.
(2) I suggest using two iTunes because I do recommend you use iTunes in conjunction with your apps, even if it is to Have the iTunes be a storage bin for your apps. I, personally, only use my iTunes to make a backup for my iPad. These backups contain all the settings, app data, etc. for your iPad. Yes, iTunes gets a bad rep (and, it's deserved, mostly). But, it's still the best iPad management tool out there. Even if you only sync it once, it'll will benefit you in the long run, should you ever need a computer to "fix" an issue with the iPad.
Of course, you two can share just one iTunes/computer. iTunes will know which iPad is connected, so you still won't have to mix up apps and songs and such (this is how I manage my mom's and my iPads). Yes, I know that Apple says you won't need a computer or iTunes to run the iPad. And that's true. But, having a synced iTunes for your iPad may be an invaluable "just in case" option. And, trust me, after 2+ years of reading this forum - I know!
So, you're ready to go. Now - all you have to do is wait for Christmas!

You can hang out here and we'll keep you company in that long wait. And, of course, we'll be here after you've opened your new shiny should you develop more questions. Have fun...
Marilyn